Carnitine (vitamin Bt; 3-hydroxy-4-trimethylammonio-butanoate) is a quaternary ammonium compound biosynthesized from the amino acids lysine and methionine. In living cells, it is required for the transport of fatty acids from the cytosol into the mitochondria during the breakdown of lipids for the generation of metabolic energy. Carnitine exists in two stereoisomers. The biologically active form is L-carnitine, whilst its enantiomer, D-carnitine, is biologically inactive.
Carnitine is widely used as a nutritional supplement in food and feed applications. However, pure carnitine is highly hygroscopic and the handling and storage is problematic. Thus for many applications it is advantageous to provide and use L-carnitine in the form of a salt.
EP 0 434 088 A1 discloses that the salt L-carnitine-L-tartrate is stable at normal air moisture during storage and can be processed without special precautions. It forms a crystalline powder, which can be easily processed and is particularly suitable for processing with rapidly running machines, since it does not tend to stick together or become lumpy. Moreover, it is odourless and has a refreshing taste due to the tartaric acid. The inventors suggest producing L-carnitine L-tartrate by adding L-carnitine crystals to a solution of L-tartaric acid in hot 90% aqueous ethanol. The method requires the use of highly pure crystalline L-carnitine, which is relatively complicated to manufacture and hygroscopic.
WO00/56701 discloses a method for preparing L-carnitine tartrate by mixing solid L-carnitine with tartaric acid in the presence of a low amount of water. The crystals are obtained after heating, cooling and grinding the mixture.
In order to improve the method of EP 0 434 088 A1, it is suggested in CN 1167669-C to obtain L-carnitine L-tartrate in a process starting from a water-containing 10% (w/v) decoloured crude L-carnitine solution. After heating the solution to a temperature between 70° C. and 78° C., a calculated dose of L-tartaric acid is added. The mixture is heated for 0.3 to 2 hours and cooled to a temperature below 10° C. After filtering, washing and drying, precipitated carnitine tartrate crystals are isolated. The inventors include a comparative example relating to EP 0 434 088 A1 and conclude that their novel process would be simpler, avoid solvents required for the production of pure L-carnitine crystals and provide higher yields.
Since L-carnitine tartrate is an important industrial product, there is an ongoing need to improve processes for its production and to render them more efficient. Since large amounts of reactants are required in such industrial processes, it is important to render such processes more efficient regarding energy consumption. Further, it is desirable to reduce the amounts of reactants and solvents and to use reactants which are easily available. CN 1167669-C requires a relatively high temperature between 70 to 78° C. as well as a relatively low temperature below 10° C., which requires cooling. Further, relatively high amounts of ethanol are necessary for the process, and the yield of L-carnitine in the best embodiment (according to the inventors) of example 3 was 89.6% in combination with an L-carnitine content of 64.6% in the crystals, which indicates that a significant amount of the starting compounds was not reacted.